Electric power steering systems comprise a manual steering system for converting a turning angle of a steering wheel to a steering angle of road wheels, and an electric motor incorporated therein to provide an assist steering torque to the manual steering system so as to reduce the effort required for the vehicle operator to turn the steering wheel. However, such conventional electric power steering systems did not take into account changes in the road condition on which the resistance to the steering effort depends, and it is known that the assist torque tended to be excessive when the frictional coefficient between the road surface and the wheels is low such as when the road surface is covered with snow. An excessive steering assist torque tends to cause an excessive steering, and creates some discomfort to the vehicle operator.
To overcome such problems or to control such an excessive steering on slippery road surfaces, it was proposed in the copending U.S. patent application Ser. No. 08/704,968, now U.S. Pat. No. 5,729,107 filed Aug. 28, 1996 and assigned to the same Assignee to control the steering assist torque of a power steering system depending on the road condition, in particular the frictional coefficient between the road surface and the wheels. The contents of this copending application are hereby incorporated in this application by reference.
However, this previously proposed system is effective only when the steering angle and the side slip angle increase or decrease in the same direction. For instance, this relation does not hold when the vehicle has gone into a counter-steer condition or, in an extreme case, into a spin due to the insufficiency of the cornering power of the rear wheels, and the vehicle operator has steered the vehicle in the opposite direction from the turning direction of the vehicle in an attempt to regain the control of the vehicle. When such a counter-steer condition exists, the previously proposed system produces a supplemental steering torque which opposes the effort to counter-steer the vehicle, and may therefore hamper the vehicle operator in his effort to carry out the counter-steer maneuver.